1. Field of the Invention
The present invention relates to a modem system for making the reception and transmission of data between the general telephone network and particularly to a modem system including a communication control function therewith.
2. Description of the Prior Art
When communication between data terminal units is to be made through the telephone network, the latter is connected with each of the data terminal units through a modem system. Usually, the modem system comprises a modulator/demodulator and a standard protocol interface.
The modulator/demodulator is used to make the analog/digital conversion between digital data handled in the data terminal units and analog signals transferred through the telephone network. Digital data from the outputs of the data terminal units are transferred to the telephone network after they have been modulated into analog data. On the other hand, analog data from the telephone network are given to the data terminal units after the analog data have been demodulated into digital data.
The standard protocol interface is utilized to convert data forms different from one data terminal unit to another into a standard protocol. Such interface is normally in the form of RS232C interface. By the use of such interface, the standardized communication can be made between data terminal units having different specifications.
However, the conventional modem systems require that the data terminal units to be connected therewith has a communication control function. This results in a defect that any data terminal unit having no communication control function cannot be connected with the modem system.
Typical data terminal units without communication control function include printers, plotters and other instruments including word processors or personal computers without communication function. Such instruments having no communication control function cannot be coupled with the telephone network unless they are modified, since data are parallel-processed therein. For example, when a printer or plotter is to be connected with the telephone network to output data from a data terminal unit on the transmitter side at the printer or plotter, the data conversion must be performed through a parallel interface such as centronix interface or GPIB interface. However, the conventional modem systems do not have such a parallel interface. Thus, the conventional modem systems cannot independently control the printer, plotter or the like.
In recent years, it is increasingly required to perform data communication between terminal units of various different types. In such a case, even if there is used data terminal units without communication function or terminal units including parallel interfaces such as centronix interfaces, it is desired to simply connect the common telephone network with terminal units standardized with RS232C interfaces such that the communication between the terminal units can be made freely through the telephone network.